Time-bandwidth reduction system for image signal transmission



5 sheets-sheet 1 C. F. TEACHER lETAL April 7, 1964 TIME-BANDWIDTH REDUCTION SYSTEM FOR IMAGE SIGNAL TRANSMISSION Filed Aug. l5, 1962 April 7, 1964 c. F. TEACHER ETAL 3,128,338

TIMR-BANOWIOTH REDUCTION SYSTEM FOR IMAGE SIGNAL TRANSMISSION 5 Sheets-Sheet 2 Filed Aug. 15, 1962 N Sl IIIIII l l l RMD. d," 6.

April 7, 1964 c. F. TEACHER ETAL 3,128,338

TIME-BANDWIDTH REDUCTION SYSTEM FOR IMAGE SIGNAL TRANSMISSION Filed Aug. l5, 1962 5 Sheecs-Shee(l 5 April 7, 1964 c:` F. TEACHER ETAL. 3,128,338

V TIME-BANDwIDTH REDUCTION SYSTEM FOR IMAGE SIGNAL TRANSMISSION Filed Aug. 15,1962 5 sheets-Sheet 4 /30 1 1 1 IMT n n JL JL 1L J\ H f 7a' Bfr-7a (2&4,

BYM) (JW /9770/9/VEY April 7, 1964 c. F. TEACHER ETAL 3,128,338

TIME-BANDwIDTH REDUCTION SYSTEM FOR IMAGE SIGNAL TRANSMISSION United States Patent O TME-BANBWIBTH REDUCTIN SYSTEM FOR MAGE SIGNAL TRANSMISSIN Charles F. Teacher, Philadelphia, and Melvin E. Partnr,

Monmomeryville, Pa., assignors to Philco Corporation, Philadelphia, Pa., a corporation of Delaware Filed Aug. i5, 1962, Ser. No. 233,044

l2 Claims. (Cl. lipid-6.8)

The present invention relates to image transmission systems and more particularly to systems including means for minimizing the time-bandwidth product required to transmit images.

It is 4conventional to transmit images from one point to another by scanning the image point by point to develop an electrical signal having an amplitude variation proportional to the ldensity variation along the scanning path. The conventional constant speed rectilinear scan has the disadvantage that the bandwidth of the transmission system is not fully utilized when scanning areas of low detail. Systems employing variable velocity scanning rasters have been proposed. However, such systems tend to be quite complex. Also it is often difficult to maintain proper registration between the scanning means at the transmitter and the reproducing means at the receiver.

VIt is an object of the present invention to provide an improved image transmission system which makes eicient use of available bandwidth.

Another object of the present invention is to provide a simple economical means for maintaining proper registration between scanning means at the transmitter and at the receiver.

In general, these and other objects of the present invention are achieved by providing a mask formed with spaced reference lines. Means are also provided for scanning the image to Ibe transmitted. The scanning of the mask and the scanning of the image occur in synchronism. Reference signals are generated as said lines on said mask are scanned. Video signals are generated as the image to be transmitted is scanned. Means are provided for detecting the presence of detail in an area ot' the image bounded in part by two adjacent reference lines. The image scanning means is caused to retrace and rescan preferably at a slower rate the area in which a difference in the video signal is detected. The video signal and the block signals generated in response to the detection of detail are combined for transmission to a remote receiver.

For a better understanding of the present invention together with other and further objects thereof reference should now be had to the following detailed `description which is to be read in conjunction with the accompanying `drawings in which FIGS. l-A and l-B together form a detailed diagram of a preferred embodiment of a transmitter arranged in accordance with the present invention;

FIG. 2 is a detailed diagram showing the relationship of the scanning beam to index and reference lines of the system;

FIG. 3 is a detailed diagram of the quantizer and high frequency detector circuit of FIG. l-A;

FIG. 4 is a detailed block diagram of the subtractor circuit of FIG. l-A;

FIG. 5 is a detail showing of a ladder network of the type employed in FIGS. l-A and l-B;

FIGS. 6-A and 6-B together illustrate the output signal generated in response to scanning certain image patterns;

FIG. 7 is a 4detailed block diagram of a portion of the circuit of FIG. lB;

3,128,338 Patented Apr. 7., 1964 ICC lutilizing the signal provided by the system of FIGS.

l-A and l-B;

FIG. 10 is a representation of the signal obtained from a second embodiment of the invention.

In the embodiment shown in the FIGS. l-A and l-B it will be assumed that fixed copy such as a black and white photographic print is scanned by a flying spot scanner and that the light reflected from the copy is picked up by a plurality of photocells. However, it is to be understood that both the copy and the scanning means may take other forms. For example, the copy may be a transparency or a scene Viewed through suitable optics. The scanning means may be a multibeam television pickup tube or the like.

`In FIG. l-A the copy to be reproduced is shown at 2@ in the upper left corner. A lens 22 images the screen of flying spot scanner 24 on copy 26. Preferably the image of the active area of the screen of ying spot scanner 24 coincides with the area of the copy 2@ to be reproduced.

A prism 26 is provided between scanner tube 24 and lens 22. As will be explained in more detail presently, prism 26 converts the single white spot on the face of scanner tube 24 to a plurality of differently colored spots on copy 20.

Two partially reflecting mirrors 28 and 30 are placed between lens 22 and copy 20. Mirror 30 causes the screen of scanner tube 24 to be imaged on a generally opaque mask 32. Mask 32 is provided with a plurality of evenly spaced, parallel, transparent reference lines. Broken lines 34 on copy 20 represent the portions of the copy which are scanned at the same times as the reference lines on mask 32.

Mirror 2S causes the screen of scanner tube 24 to be imaged on a second mask 36 which is provided with horizontally disposed transparent index lines. Again, broken lines 38 on copy 20 represent the portions of the copy which bear the same relationship to the scanning element as the lines on mask 36.

A single photocell 4d is provided for registering the intensity of the light passing through mask 32. Each of two photocells 42 and 44 views the entire area of mask 36. A red filter yt6 is interposed between mask 36 and photocell 42 and a blue filter 4S is interposed between mask 36 and photocell 44.

Three additional photocells 5), 52, and 54 are positioned so that each receives light reflected from the entire area of copy Ztl which is to be reproduced. Photocells 50, 52 and 54 are provided with a red `filter 56, a green filter 58 and a blue filter 60, respectively, so that each photocell responds to light of one color.

Reference should now be had to FIG. 2 for a more detailed showing of the relationship between the spot provided by scanner tube 24, prism 26, lens 22 and the reference and index lines provided by masks 32 and 36. As shown in FIG. 2 the elliptical spot A62 occupies substantially the entire space between the index lines 38 established by mask 36. The prism 26 causes the upper end of spot 62 to have a predominantly red color, the center potrion to have a predominantly green color, and the lowermost portion to have a predominantly blue color. It is to be understood that the reference to lower-most and upper-most are employed only for identication in the drawing. Since photocells 50, 52 and 54 respond only to red, green and blue signals, respectively, it will be seen that the combination -just described provides the equivalent of three simultaneous scans along adjacent, parallel paths on copy 20'. Other means for scanning three adjacent parallel paths may be substituted for the means shown. For example, scanner 2d may provide three parallel beams, each intensity modulated at a dif-ferent frequency. Prism 26 may then be omitted and optical lters 56, 53 and 60 replaced by suitable electrical iilters connected to the outputs of phototubes 50, 52, and SLi.

The vertically disposed reference lines 34 are spaced apart by substantially the same distance as horizontally disposed index lines 38. Therefore the image on copy is, in effect, divided into blocks, each block being deiined by two lines 34 and two lines 3S and including nine picture elements.

The signals from the three photocells 50, 52 and 5d pass through individual multilevel quantizers which are collectively represented by block 66 in FIG. 1-A. The individual quantizers are represented by blocks 613-70 in FIG. 3. Each of the quantizers 63-70 may be an analog to digital converter followed by a digital to analog converter. Single circuits for accomplishing multilevel quantization are also known in the art. The outputs of individual quantizers 6%-70 are supplied to the inputs of delay means 72, 73, '74, respectively. Delay means 772-74 provide substantially equal delays. In the example illustrated in FlG. l-A it will be assumed that these delays are equal to the time required to scan from one reference line 3d to the next at the normal rate of scan of the system. The outputs of delay means 72-74 are connected by way of gates 82, 83, and gli to the three inputs of an adder circuit S6. As will be explained in more detail presently, only one of the gates i12-84 is conditioned for the passage of a signal at any time.

The output of adder circuit 36 is connected to a sarnpling and storage circuit S8 which may take the form of a conventional bcxcar circuit. The output of circuit SS is connected to a signal combiner circuit which also receives inputs from three one-shot ip-ops 92, 93, and 94. As will be explained in more detail presently, iiipops 9294 are energized by the synchronizing circuits of the system of FIGS. 1A and l-B.

A source of periodic signals in FIG. l-B provides the main clock or timing pulses for the circuit of FIGS. l-A and 1-B. These main clock pulses are supplied by way of a normally open gate circuit 132 to the input of horizontal counter 134. Counter 134 may be a binary counter having suiiicient stages to count to at least 1600. The several outputs of counter 134 are connected to a ladder network 136. Ladder network 136 is shown in more detail in FIG. 5. It comprises a plurality of branches m, 138k, 138e, etc. which are connected between terminal 139 of a source of fixed potential represented by the minus sign and a common terminal 141). Each branch comprises a resistor and the emitter-collector circuit of a transistor. The base connection of each of transistors 111451, 141411, 14de, etc. is connected to a respective one of the output taps of horizontal counter 134. The resistors 14251, 142k, 142C, etc. have different values of resistance, each value oi resistance being representative of the count registered by the corresponding tap on counter 134. Common terminal is also the emitter terminal of a grounded-base emitter follower circuit which includes grounded base transistor 146 and a resistor 141%. Thus the signal present on output 150 of the ladder network 136 is a voltage which increases in linear steps at a rate controlled by the main clock 130. The signal present on output 151i of ladder network 136 is supplied by way of adder circuits 152 of FIG. l-B and 154 of FIG. l-A to the horizontal deiiection coils of deriection yoke 156 of flying spot scanner 24.

Selected outputs of horizontal counter 134 are also connected to a matrix circuit 15S. Matrix circuit 153 provides an output signal when horizontal counter 134 has reached a predetermined count. Matrix circuits of this type are well known in the art, but in the interest of cornpleteness a more detailed showing of the connection between counter 134 and matrix 158 is shown in FIG. 7. As shown in FIG. 7, the outputs of horizontal counter 13d which represent the counts of 26, 29, and 210 are connected to the three inputs of an and gate 162. A signal is provided at output 164 of and gate 162 only when the three outputs of horizontal counter 134 just mentioned are energized. These three outputs are energized when counter' 134i reaches the count of 1600.

The output of matrix 156 is connected to the reset input oi counter 13d and to the signal input of vertical line counter 166. The outputs of vertical line counter 166 are connected to the inputs of a second ladder network 168. Ladder network 168 supplies a stepped deiection signal to the vertical deflection coils of yoke 156 by way of adder circuit 170 of FIG. lA.

Selected outputs of vertical counter 166 are connected to the inputs of a second matrix circuit 172. The output of matrix circuit 172 is connected to the reset terminal oi counter 166. Matrix 172, vertical line counter 166, and ladder network 166 may be similar to matrix 158, horizontal counter 134 and ladder network 136, respectively.

Mask 36, iilters d6 and 18, and photocells 42 and 44 form part of a vertical de'lection correction or indexing system. The outputs of photocells d2 and 44 are connected to a subtractor circuit 173. The output of subtractor circuit 173 is supplied by way of low-pass iilter and amplifier circuit 174 to a second input of adder circuit 170.

Mask 32 and photocell 40 form a horizontal reference system. The pulse signals supplied by photocell 40 are supplied to the input of normally open gate 182 of FIG. l-B. Gate 132, counter 1041-, ladder network 186 and subtractor circuit 137 form a position control or indexing system for the horizontal sweep. Counter 134 and ladder network 186 may be similar to horizontal counter 134 and ladder network 136, respectively. It the horizontal sweep is proceeding at the proper rate in the horizontal direction under control of signals from main clock 130 the output of ladder network 136 will be identical to the output ot ladder network 186. Under the condition just mentioned there will be no output from subtractor 187. 1f the horizontal sweep is not proceeding exactly :according to the voltage produced by ladder network 136, due to irregularity in the deliection coil or the like, the output signals from ladder network 136 and 1&6 will not be equal and subtractor 137 will provide a correction signal to a second input of adder circuit 152. As shown in FIGS. l-A and 1-B, the output signal ot adder 152 is supplied by way of adder circuit 154 to the deflection yoke 156.

The scanning circuits just described operate `as follows. Main clock 130, horizontal counter 134 and ladder network 136 produce appropriate potentials for causing the beam of the iiying spot scanner to trace a horizontal path across copy 20. Gate 182 is normally operative toy pass a signal. Photocell dil generates a pulse each time the iiying spot scanner passes a point corresponding to one of the reference lines 34. Any deviation of the horizontal sweep from its intended value will result in a diderence between the outputs ot ladder network 136 and ladder network 136. The correction signal supplied by subtractor 137 corrects the position of the spot 62 on copy 20.

At the end of the horizontal sweep, that is, at the end of a predetermined number of clock pulses, matrix 158 provides a reset signal to horizontal counter 134 which resets counter 134 to zero and initiates the start of the next horizontal sweep. The output of matrix 158 is also supplied to the input of vertical line counter 166. Vertical line counter 166 and ladder network 168 produce a vertical sweep voltage which advances the position of the spot 62 on copy 20 one block in the vertical direction for eacn horizontal sweep.

If the spot 62 is deviated upwardly from its intended position for any reason, the red portion of the spot will coincide with one of the transparent index lines 318 on mask 36. Under these conditions photocell 42 generates an output signal but photocell `44 does not. Subtractor circuit 173 provides a correction signal by way of low pass ilter and amplifier |174 to Iadder circuit 170. The polarity of this correction signal is such as to restore the spot to its intended path on copy 20. Conversely, if the spot 62 is displaced downwardly from its proper path, photocell 44 generates -an output signal while photocell 42 does not. Therefore a correction signal of the opposite polarity is supplied to adder circuit .170.

At the end of a preselected number of vertical lines, matrix 172 provides a reset signal to vertical counter 166. This returns the spot 62 to the position it occupied at the start of the scan.l Since only one complete scan of the copy is required 'for the proper transmission of the im-age, the output signal of matrix 172 may be supplied to means (not shown) for rendering the system inoperative or for advancing lanother piece of copy to the position occupied by copy 20.

Dur-ing normal scan, gate 82 of FIG. l-A is conditioned to pass a signal while gates 83 and `84 are conditioned to block signals. Thus the signal supplied by photocell 50 is passed through quantizer 66, delay circuit 72, gate 82, adder circuit |`86 'and circuits 88 and 90 to the output connection 91 of the system. 'Ihe output 91 of the system may be supplied to a suitable signal transmission device such as a wire line or -a radio transmitter. As will be explained in more detail presently, normal scan is employed only if there is no substantial variation in density within the block being scanned. =If there is no substantial u variation in density the three signals supplied by photocells 50-52 are substantially lalike and only one need be transmitted.

The pulse signals provided -by photocell as the spot 62 progresses across the reference lines 34 are also supplied by delay means 176 to sample and storage circuit 88. Upon receipt of a signal from delay means 176, circuit 88 vis reset to register the amplitude signal currently being supplied by gate `82. This amplitude is maintained until the next signal is supplied by delay circuit 176.

The remainder of the system shown in FIGS. l-A and 14B `comprises means for detecting the presence of detail in the block being scanned :and changing the scan characten'stics accordingly. This is accomplished by detecting the presence Iof high frequency components in the signal supplied by any one of the photocells 50, 52, and 54 and/ or detecting any diierences in amplitude of the signal supplied by these three photocells `50, 52 and 54. High frequency components in the signals provided by photocells 50, 52, and 54 represent relatively rapid changes in density `along ta horizontal line. These high frequency components are detected by high frequency detector 78. This circuit is shown in more detail in FIG. 3. As shown in FIG. 3, circuit 73 comprises three high pass lters 202, 203, and 204 connected to the outputs of quantizers 68, 69 and 70, respectively. Filters 202-204 are connected by way of threshold circuits 203, 209 and 210, respectively, to three inputs of or circuit 206. Or circuit 206 provides a signal on output connection 212 if one or more of the filters 202-204 provides an output signal which exceeds the threshold level of the associated threshold circuit 208-210. As shown in FIG. `l-A, output 212 is connected to one input of 'a gated or circuit 214.

Subtractor circuit 76 provides means for detecting the presence of a difference in the amplitudes of the signals supplied by photocells 50, 52 and 54, respectively. Subtractor circuit 76 is shown in more detail in FIG. 4. Subtractor unit 216 of FIG. 4 receives an input from quantizer 68 and -a second input from quantizer 70 of FIG. 3 and provides an output signal there is `any difference between these two input signals. Similarly, subtractor unit 218 receives input signals `from quantizer 70 and quantizer 69, and provides an output signal if there is any diierence between the two input signals.

manner.

Absolute value circuits 220 and 222 provide output signals 'which are dependent only -on the amplitude and not the polarity of the output signals of subtractor units 216 Iand 218. T'wo inputs of or circuit 224 are connected to the outputs of absolute value circuits 220 and 222, respectively. Thus or circuit 224 will provide a signal at output 226 if there is any difference in amplitude between the signals supplied by quantizers 68-70.

As shown in FIG. l-A, output 226 of subtractor 76 lis supplied to a second input of gated or circuit 214. YO-r circuit 214 provides an output signal if either input 212 or input 226 is energized. 'Ihis signal is `generated in time coincidence with la timing pulse lfrom clock 130. Or circuit 214 may include `suitable signal storage means, such las a capacitor, to maintain the signal supplied by circuit 7 6 or 78 until the next clock pulse occurs.

.The output of or circuit 214 is connected to flipflop 22S to control the setting of this iiipdiop. When iiip-op 228 is in the se condition in response to a signa-l `from or circuit 21-4, gates 132 and 182 of FIG. l-B are rendered inoperative to pass signals. Thus the horizontal and vertical deections of the beam of scanner 24 are yfrozen at their .pre-attained values.

The signal present at output 229 of ilip-flop 22S causes normally closed gate circuit 230 to be conditioned to pass signals received from photocell 40. The output of gate circuit 230 is connected by Way of diierentiator 232, negative clipping diode 234 and one-shot flip-flop 236 -to the input of counter circuit 2.38. `Counter circuit 238 may be a conventional counter 'circuit in which output connections a through g are energized in sequence as spaced pulse signals are supplied by flipdlop 236. =Nega tive clipper 234 insures that only one pulse will be supplied -by ip-ilop 236 for each pulse supplied by photocell 440.

The function of counter 238` is to control the rescanning sequence of ying spot scanner 24 when `a signal `from circuit 214 indicates that the block just scanned includes detail which will not be reproduced under normal scan conditions. In gene-ral, counter 238 causes the spot 62 to make three rapid retraces and three slow iforward scans between two of the lines 34. This is `accomplished in the following manner. The outputs b, d, and f of counter 238fare connected to three inputs of or circuit 242. The -output connection of or circuit 242 is connected to the control input of normally blocked gate circuit 244. A clock pulse source 246 is coupled to the signal input of gate 244. Preferably clock pulse 246 provides pulses yat the repetition rate of main clock circuit of FIG. l-B. 'Ihe -output of gate 244 is connected to one input of a reversible counter 250. Reversible counter 250 :may be similar to counters 134 and `166 of FIG. 1-B except that counter 250 will count in one direction in response to signals supplied in input 252 and will count in a reverse direction in response -t-o input sign-als supplied to input 254. A count of three is required in the present example. Therefore counter 250 may be provided 'with three outputs which are energized in sequence.

A gate circuit 258 couples the output of a third periodic clock signal source 260 to input 254 of counter 250. Outputs a, c, and e of counter 238 are connected to three inputs of or circuit 262. The output of or circuit 262 is connected to the control input of gate 25S. Fast clock source 260 provides a signal having a frequency several times that of the clock source 246.

Retrace and rescan are accomplished in the following Output 229 of flip-flop 228 is also coupled to the Reset input of counter 238 by way of ditferentiator 264 and diode clipper 266. Counter 23S is reset when ip-op 228 is set by circuit 214. In its reset condition output a is energized. Reversible counter 250 is normally at its maximum count. The signal on output a of counter 238 permits fast clock pulses from source 260 to be supplied to reversible counter 250. The amplitude of the signal present on output 272 of ladder network 270 will decrease as the count registered by counter 250 decreases. This will result in spot 62 moving to the left. This movement will continue until spot 62 coincides with one of the index lines 34. That is, spot 62 will retrace one whole block at a rapid rate. When .spot 62 coincides with one of the reference lines 34,

photocell 40 supplies a signal by way of gate 230, differentiator 232, clipper 234 and one-short ip-op 236 to counter 238. This causes output a to be de-energized and output b to be energized. With output a de-energized the signals from clock 260 are no longer supplied to input 254. Instead gate 244 permits signals from source 246 to be supplied to input 252 of counter 25h. This causes counter 25) to increase its count and thereby increase the amplitude of the signal at output 272. Spot 262 is then deflected to the right at a rate equal to one third the normal scan speed.

When spot 62 again coincides with one of the reference lines 34 a signal is again supplied by photocell 4t) which results in counter 238 being stepped to the next highest count. That is, output c is energized and output b is de-energized. With output c energized the reversible counter is again returned to a lower count and spot 62 is again deflected to the left. Spot 62 continues to oscillate between two lines 34 as outputs d, e and f of counter 233 are energized. At the end of the third sweep to the right, the signal generated by photocell 40 causes output g to be energized. This resets ilip-ilop 22S and deactuates gate 230. At the Sametime gates 132 and 182 of FIGURE 1-B and gate S2 of FIG. 1-A are conditioned to pass signals. Normal sweeping operation is then resumed with reversible counter 251) maintained at its high count condition.

Each of the photocells Si), 52 and 54 supplies a continuous output signal. However, during normal scanning operations only gate 82 is conditioned to pass a signal. The activating signal for gate S2 is supplied by output g of counter 238 by Way of or circuit 240. Since normal scan is employed only when there is no Variation in density within the block, the signals provided by all three photocells 5t?, 52 and 54 will be equal and only one need be transmitted.

Gates S2, 83 and 34 are conditioned to pass signals on the first, second and third slow rescans, respectively, which follow the generation of a signal by circuit 214. This is accomplished by coupling outputs d and f of counter 238 directly to gates S3 and 84 and by coupling outputs b to gate 82 by way of or circuit 24).

The output of or circuit 214 is also connected to an input of single-shot llip-iiop 92. Flip-1101392 supplies a synchronizing pulse to signal combiner 91B to indicate that a change in the mode of scan is being initiated. The signal provided by iiip-fiop 92 will he identified hereinafter as a block pulse. The signal provided by lip-flop 92 may have duration of one clock pulse interval. The output of matrix S and the output of matrix 172 are connected to single shot lip-iops 93 and 94, respectively. Single shot ip-iiop 93 provides an end of line pulse to signal combiner 90 While single shot Hip-flop 94 provides an end of frame pulse to signal combiner 90. The signals provided by flip-flops 93 and 94 should differ from each other and from the signal provided by ipop 92 in amplitude and/or time duration so that the receiver may identify the nature of the information being transmitted. All three flip-hops 92-94 provide signals having amplitudes greater than the maximum signals provided by adder 86. This permits separation of the sync signals from the video signals by simple clipper circuits.

The operation of the system of FIGS. l-A and l-B will now be explained with reference to the diagram of FIG. 6. FIG. 6-A is a portion of copy 20. The variations in density shown in FIG. 6-A have been arbitrarily chosen to illustrate all modes of operation of the system. FIG. 6-B is a time versus amplitude plot of the signal which will appear at the output 91 as the copy shown in FIG. 6-A is scanned. In FIG. '6-A it will be assumed that synchronizing pulses 'have a relative amplitude of 10, white areas a relative amplitude of 8, Vlight gray areas a relative amplitude of 6, medium gray areas a relative amplitude of 4 and dark gray areas a relative amplitude of 2.

Pulse 362 represents a synchronizing pulse and may, for example, represents an end of line pulse. -Pulses 306, 398 and 310 of FIG. 6-B represent areas 366' 308 and 310 in FIG. 64A. Each of these `three .pulses represents the density of all nine picture elements of the corresponding block in FIG. 6-A. Pulse 310 has a. smaller amplitude than pulses 3% and 308 since area 210' is darker than areas 3Go and 308.

Pulse 312 is a block ,pulse generated by single shot iiip-op 92 in response to a signal from or circuit 214. The presence of this pulse indicates that an output lhas occurred from either high 'frequency detector circuit or subtractor 76 or both. In the example of FIG. 6 the signal is provided by high frequency detector 7S only since block 312 contains variations in density in the horizontal direction but no variations in density in the Vertical direction. Since detail has been detected in the block 312', the entire block is rescanned at a slow speed. Pulses 31221, 312b and i12c represent the signals supplied by photocell 50 during vthe rst rescan of the block. These signals are passed by red gate '32 to adder circuit 86. The next three pulses 312d, 312e, and 312i represent the signals generated by photocell 52 on the second rescan of block 312. These signals are passed 'by green gate 83 to adder circuit 86. Similarly vthe last three pulses 312g, 31211 and 312i represent the pulses generated by photocell 54 and passed by gate 84. It will be understood that signals corresponding to pulses 312%-312i are generated three times in succession, i.e. once on each rescan. Only the last group is permitted to pass gate S4.

At the termination of the third rescan, which produces pulses 3135-3121, counter 23S of FIG. 1-A restores the system to normal scan condition. However, as shown in FIG. 6-A, the next block 314 produces an output from subtractor circuit 76. Block pulse 314 is generated by flip-flop 280 to indicate the or circuit 314 again has an output. Block 314 -is rescanned at a slow speed. The resulting pulses are represented by the nine pulses Shia-314i. following pulse 314. Again pulses SME-314C are passed by gate '82, pulses 3141-314f are pasesd by gate 83 and the final three pulses 314f=-314i by gate 84. At the end of the third rescan of block 314 normal scan is resumed.

Block 316' is again represented by a single pulse 316 since there is no variation in density throughout block 316.

Block pulse 318 indicates that detail is detected in block 318'. In this instance both high frequency detector 78 and subtractor 76 have outputs. Again, the nine pulses 31321-318i following block pulse 318 represent the outputs of photocells 50,152 and 54 as passed'sequentially by gates S2, 83 and 84. The final block 320 is represented by a single pulse 320 since there is no variation in density in this block.

FIG. 8 is a time versus amplitude plot of certain signals present at selected points in FIGS. 1-A and 1-B. The waveforms of FIG. 7 have been given prime reference numerals corresponding to similarly numbered blocks in FIGS. l-A and l-B. Thus wave form 214 of FIG. 8

`represents the output signal of gated or circuit 214 of FIG. l-A, etc. The waveforms of FIG. 8 represent the signals present during the scan of a block such as block 318 of FIG. 6-A.

While the present invention is concerned mainly with 'means for transmitting an image, a block diagram of a typical receiver is shown in FIG. 9 to illustrate the manner in which the signal generated by the system of FIGS. l-A and l-B may be utilized. The signal input from the data link is shown at 350. The data link may be any suitable means for coupling output 91 of FIG. l-A to input 350 of FIG. 9. Input 350 is coupled to a quantizer and sampler circuit 352 which removes minor variations in amplitude introduced in the data link. The output of circuit 352 is connected by way of amplifier 354 and the three gates 355, 356 and 357 to three inputs of a display recorder 360. Display recorder 360 may be a triple gun cathode ray tube. It is not necessary that this tube have a color screen since the red, green and blue signals represent only different lines on the image and not different colors of the original image. Display recorder 360 preferably has means such as a direct view storage tube for maintaining the image during time of transmission of the entire frame. Alternatively a conventional cathode ray tube having a photographic iilm positioned to be exposed by the screen thereof may be employed. The latter system will generate a permanent record of the image.

A sync separator circuit 362 is also coupled to input 350. Sync separator circuit 362 may be a conventional bottom clipping circuit which separates out the high amplitude sync pulses from the remainder of the signals.

Scan control 364 may be any suitable circuit for generating the deflection signals for display recorder 360. For example, they may be similar to the sweep circuits shown in FIGS. l-A and 1-B. End of line and end of frame sync pulses supplied by Way of connection 366 will cause the scan control 364 to generate appropriate signals to form a rectangular raster similar to the normal sweep at the transmitter. The presence of a block control signal on connection 368 will cause the normal sweep to be interrupted and the next following block to be scanned three times at a rate one third the normal rate.

A gate generator 370 also receives block pulses from connection 368 and generates three sequential gates corresponding generally to three series of gating pulses, each series corresponding generally to wave forms 240 and 238b and 238f of FIG. 8. These gating pulses cause gates 355, 356 and 357 to be gated on in sequence during the slow scan. All three gates 355, 356 and 357 are conditioned to pass signals during normal scan so that simultaneous recording of the video signal on all three lines is accomplished. A pulse Shaper 374, which receives an input signal from connection 368, supplies blanking signals to blanking amplifier 354. This blanking amplifier 354 prevents the block pulses from being recorded on display recorder 360.

The operations of scan control 364 and quantizer and sampler 352 are synchronized by a clock circuit 376. Clock circuit 376 is synchronized with the transmitter by means of synchronizing signals supplied to clock circuit 376 by way of connection 366.

As shown above, the system of FIG. 1 will transmit the 72 picture elements of FIG. 6-A in 35 pulse intervals. The system of FIG. 1 is capable of operating with more sophisticated codes with a still further reduction in the time-bandwidth product. FIG. shows a code which may be employed with only minor modifications of the circuit of the FIGS. l-A and 1-B. This pattern shown in FIG. 10-A is identical to the pattern of FIG. 6-A. The code shown in FIG. IO-B takes advantage of line to line correlation in both horizontal and vertical directions as Well as element to element correlation. Blocks in which all elements have identical density are transmitted as a single brightness pulse as before with the same advantage that the level may change between blocks. Thus blocks 306', 308', and 310 are represented by pulses 306, 308 and 310 as before. A block such as block 312, which contains high horizontal frequencies but no differences in the vertical direction, will produce an output from the high frequency detector 78 but no output from subtractor 76. It will be assumed that a single interval block pulse 380 is generated when this condition exists. In response to an output from the high frequency detector 78 block 312 will be rescanned at one third normal speed but only It) once. At the receiver the speed of scan will be reduced to one third normal, but all three gates 355-357 will remain open, i.e. will pass signals.

A block such as block 314' which has no variation in the horizontal direction but differences in the vertical direction will produce an output from subtractor 76 but no output from high frequency detector 78. It will be assumed that a two interval block pulse 384 is generated when this condition exists. In response to the output signal of the subtractor 76 alone the beam may be stopped at the end of the block and gates 82, 83 and 84 gated on in sequence without rescanning the beam. Alternatively the block 314 may be rescanned three times at the fast speed. Again gates 82, 83 and 84 are conditioned to pass signals in sequence. The signals produced by either operation are shown as pulses 386% 3861, and 386, respectively.

Block 316' is again transmitted as a single pulse 316. No block pulse is necessary.

A block such as block 318 will produce an output from both high frequency detector 78 and subtractor 76. It will be assumed that a three interval block pulse 388 is generated when this condition exists. In response to a signal indicating an output from both subtractor 76 and high frequency detector 78, the entire block is rescanned at slow speed in the manner previously described. Thus the series of video pulses generated to represent the individual elements of block 318 will be identical to those shown in FIG. 6B.

Block 320 will again be represented by a single interval pulse 320 which is identical to pulse 320 of FIG. 6.

Although twelve intervals are required to transmit the nine elements of a block such as block 218', the code chosen is still superior to that of FIG. 1 in time-bandwidth reduction. That is, the 72 picture elements of FIG. 10-A are transmitted in a total of 26 intervals. The circuit of FIG. 1 requires 35 intervals. Modification will be required in the end-of-line and end-of-frame pulses generated by flip-flops 93 and 94 since two and three interval block pulses are now employed. The end-of-line and end-of-frame pulses may take the form of four and iive interval pulses or combinations of one interval pulses spaced not more than one interval apart. It will be seen that the one interval block pulses cannot be spaced closer than four intervals apart in the system just described.

The sync pulses and block pulses may be separa-ted from Ithe video signals at the receiver -by amplitude selection as before. The nature of each block pulse or synohronizing pulse may be determined by suitable pulse width discriminator circuits.

While the invention has been described with reference to what is or presen-t considered to be the preferred ern bodiment thereof, it will be `apparent that 'various modifications and other embodiments thereof will occur to those skilled in the art within the scope of the invention. Accordingly we desire the scope of our invention to be limited only by the appended claims.

What is claimed is:

1. In an image scanning system adapted to receive an image to be scanned, the combination comprising a mask formed with spaced reference lines, image scanning means including a movable image scanning element, scan control means coup-led to said image scanning means for causing said image to be scanned at a first rate, said image scanning means including means -for generating a video signal representative of the density of the image. along the path scanned, means associated 'with said image scanning means for causing said mask to be scanned in a direction transverse to said reference lines in synchronism 'with the scanning of said image, reference signal generating means associated fwith said mask for generating position reference signals 4coincident in time with the scanning of reference lines of said mask, detail detector means responsive -to said video signal for producing signals indicative of the presence of image detail in an area delineated in part by the positions at which adjacent reference lines are scanned, means responsive to said signal provided by said detail detector means and said position reference signals for causing said image scanning means to retrace and -rescan a portion of said image between positions corresponding to positions at which adjacent reference lines on said mask are scanned.

2. In an image transmission system adapted to receive an image to be scanned, the `combi-nation comprising a mask formed Iwith spaced reference lines, image scanning means including a movable image scanning element, scan control means coupled to said image scanning means for causing the image to be scanned at a rst rate, said image scanning means including means for generating a video signal representative of the density of the image along the path scanned, means associated with said image scanning means for causing said mask to be scanned in a direction transverse to said reference lines ,in synchronism with the scanning of said image, reference signal generating means associated with said mask for generating position reference signals coincident in time with the scanning of reference lines of said mask, detail detector means responsive to said video signal for producing block signals indicative of the presence of image detail in areas delineated in part by the positions at which adjacent reference lines are scanned, means responsive to said signal provided by said detail detector means and said position reference signals for causing said movable image scanning element to retrace and rescan portions of said image between positions corresponding to positions at lwhich adjacent reference lines on said mask are scanned `and signal combining means coupled to said image scanning means and said detail detector means for combining said video signals and said block signals.

3. In an image transmission system adapted to receive an image to be scanned the combination comprising a mask formed with spaced reference lines, image scanning means including a movable image scanning element, scan control means coupled to said image scanning means for causing said image scanning element to scan the image to be .transmitted at a iirst rate, said image scanning means including means for generating a video signal representative of the density of the image along the path scanned by said scanning element, means associated with said image scanning means for causing said mask to be scanned in `a direction transverse to said reference lines in synchronism With the scanning of said image, reference signal generator means associated with said mask `for generating position reference signals coincidence in time with the scanning of reference lines of said mask, detail detector means for detecting the presence of components of said video signal which have a frequency above a preselected frequency, means responsive to said last mentioned detail detector means and said position reference signals `for causing said movable scanning means to retrace and rescan at a rate different from said rst rate portions of said image between positions corresponding to positions at which adjacent reference lines on said mask are scanned, block signal generator means responsive to the output of said detail detector means for generating a block signal upon inception of said retrace and rescan phase, and signal combining means coupled to said image scanning means and Isaid block signal generator means for combining said video signal and said block signals.

4. A combination in accordance with claim 3 wherein said portion `of said image is rescanned at a rate slower than said iirst rate.

5. In an image transmission system .adapted to receive an image to be scanned, 4the combination comprising a mask formed with spaced reference lines, image scanning means including a movable image scanning element, said image scanning element being formed with a plurality of portions for scanning a plurality of parallel paths on said image, scan control means coupled to said image scanning means for causing said image scanning element to scan the image to be transmitted at a iirst rate, said image scanning means including means for generating a plurality of video signals, each video signal being representative of the density of the image along a respective one of said parallel paths scanned by said scanning element, means associated with said image scanning means for causing said mask to be scanned in a direction transverse to said reference lines in synchronism with the scanning of said image, reference signal generating means associated with said mask `for generating position reference signals coincident in time with the scanning of reference lines of said mask, means coupled to said image scanning means for detecting the differences in amplitude, if any, in the video signals generated in the interval between the traversal of `adjacent ylines of said mask, means responsive to lthe `output signals of said last mentioned means and said position reference signals for causing said movable image scanning element to retrace and rescan a portion of said image between positions corresponding to positions at which adjacent reference iines on said mask are scanned, and gated signal coupling means coupled to said image scanning means for selectively passing or suppressing video signals generated Ithereby.

`6. The combination in accordance with claim 5 wherein said means coupled to said image scanning means for detecting dierences in amplitude in the video signals detects the differences in amplitude if any between video Vsignals representing different paths.

7. The combination in accordance with claim 5 wherein said 4means coupled it-o said image scanning means for detecting diierenccs in amplitude in the video signals detects changes the amplitude of video signals representing one path.

8. The combination 'accordance with claim 5 wherein said means coupled to said image scanning means for detecting diierences in amplitude in the video signals detects both differences in amplitude between video signais representing different paths and changes in amplitude of video signals representing 'one paith.

9. In an image transmission system, the combination comprising a mask formed with spaced reference lines, image scanning means including a movable image scanning element, scan control means for causing said image scanning means fto scan la plurality ci parallel paths on said image at la rst rate, said image scanning means including means providing yat a plurality of outputs a plurality of video signals, each video signal being representative of density of the image along `a respective one of said parallel paths, means associated with said image scanning means -for causing said mask to be scanned in a direction transverse rto said lines in synchronism with fthe scanning of said image, reference signal genenaitiing means ass-ociated with said mask for generating position reference signals coincident in time with the scanning ci reference lines 4of said mask, means coupled to said image scanning means for detecting differences in amplitude of the video signals generated in the interi/al lbetween the scanning of adjacent lines of said mask, means responsive to the output signals of said last mentioned means and said position reference signals rior causing said image 'scanning means to retnace -a-nd rescan a portion of said image between positions corresponding to positions at which adjacent reference lines on said mask lare scanned, a plurality of gated signal coupling means, each :said coupling means being associated with Aa respective one of said plurality of outpnts, control means coupled to said coupling means, said control means normally causing one of said coupling means to be conditioned to pass a video signal and the remainder of said coupling means to be conditioned to block video signals, -means coupled to said control means for conditioning `a different one of said coupling means` to pass a signal on each rescan of a portion of said image, signal combining means couple-d to said coupling means for combining video signals passed by said coupling means and means for supplying to said signal combining means 13 signals indicative of the times Kat which said retnace and reseau phase is initiated.

10. The combination in accordance with claim` 9, said combination `further comprising a second mask, means for causing said second mask to be scanned in synchronism with the scanning of said image, said second mtask being formed with index lines parallel to [the normal path of said scan of said second mask, means respons-ive .to the scanning of an index line of said second mask for correcting the position of scanning paths on said image.

1l. The combination in accordance with claim 9, Wherein said image scanning means includes a flying spot scanner, Ian optical system including a prism for imaging the screen of said iiying spot scanner on the image to tbe transmitted, Iand wherein said means providing tat 'a plurality of outputs a pluiiaiitty of video signals comprises a plurality of photo responsive elements, each elemtent being respon-sive to light of a diiierent Wave length.

lli

12. The combination in accordance with claim 9, said combination funther comprising receiver means, means for tnansm-itting signals from .said signal combining means to said receiver means, means att said receiver means for recording on .a medium information indicative .of received signal ampli-tudes .in terms oi image density, said information being recorded 'along a plurality of parallel paths, means for causing identical information to he recorded simultaneously in each of said paths in the absence of received block signals land in response to received video signals, means responsive to Ireceived block signals for causing said recording means Ito rescan Ia selected pontion of said rrecording medium a plurality of times, and means lfor initiating recording in :a different one of said recording paths on eac-h rescan.

No references cited. 

1. IN AN IMAGE SCANNING SYSTEM ADAPTED TO RECEIVE AN IMAGE TO BE SCANNED, THE COMBINATION COMPRISING A MASK FORMED WITH SPACED REFERENCE LINES, IMAGE SCANNING MEANS INCLUDING A MOVABLE IMAGE SCANNING ELEMENT, SCAN CONTROL MEANS COUPLED TO SAID IMAGE SCANNING MEANS FOR CAUSING SAID IMAGE TO BE SCANNED AT A FIRST RATE, SAID IMAGE SCANNING MEANS INCLUDING MEANS FOR GENERATING A VIDEO SIGNAL REPRESENTATIVE OF THE DENSITY OF THE IMAGE ALONG THE PATH SCANNED, MEANS ASSOCIATED WITH SAID IMAGE SCANNING MEANS FOR CAUSING SAID MASK TO BE SCANNED IN A DIRECTION TRANSVERSE TO SAID REFERENCE LINES IN SYNCHRONISM WITH THE SCANNING OF SAID IMAGE, REFERENCE SIGNAL GENERATING MEANS ASSOCIATED WITH SAID MASK FOR GENERATING POSITION REFERENCE SIGNALS COINCIDENT IN TIME WITH THE SCANNING OF REFERENCE LINES OF SAID MASK, DETAIL DETECTOR MEANS RESPONSIVE TO SAID VIDEO SIGNAL FOR PRODUCING SIGNALS INDICATIVE OF THE PRESENCE OF IMAGE DETAIL IN AN AREA DELINEATED IN 